The present invention relates to press brakes, and specifically to those in which the upper ram is moved downward toward the press bed by actuators at its ends. By the present invention, the lower die is mounted not on the bed, but on an inserted upward-stroke mechanism. The ram is then used only for preliminary positioning and clamping the workpiece; the bending stroke is performed by the insert mechanism. Its stroke length is so precise that any desired degree of bend may be formed with repeatability; so that a series of different bend angles may be programmed.
In conventional press brakes, an upper ram carrying a male die is lowered toward a fixed lower beam or bed bearing a female die, either by pitman arms or by a pair of linear actuators mounted at the ends of the ram. The depth of penetration of the male die into the female die determines the angle of the bend. No provision is normally made to assure repeatability of depth of penetration.
When heavy metal workpieces are formed, the upper ram and lower bed tend to deflect apart, this deflection being greatest at their longitudinal center, and may become permanently deformed. As a result (either of elastic deflection or permanent deformation) the angle of the bend in a long sheet metal member will vary along its length, being less at the center.
To overcome the problem of deflection at the center and make it possible to program a series of bends in the same workpiece, at least one manufacturer of press brakes, Promecam, Saint Denis, France, utilizes a movable lower ram driven at its center by a vertically-acting hydraulic cylinder, whose movement can be precisely programmed to perform a series of bends to different angles. As described in its U.S. Pat. No. 3,007,508, the center drive causes the lower ram to deflect upward at its center or "crown", compensating for the upward deflection at the center of the upper beam. A lower ram type press brake, manufactured by Fabco, Inc., Lewisville, Tex., utilizes a hydraulic-actuated wedge arrangement, operating in the vertical plane of the ram, to drive the ram upward. Others have used wedge arrangements to provide "crown" or to compensate for other inaccuracies in the ram, such as shown in U.S. Pat. Nos. 2,199,864, 2,456,856 and 3,965,721, or to adjust the level of a tool, as in U.S. Pat. Nos. 3,844,156 and 4,137,748.